This study addresses molecular constraints in the immune recognition of protein antigens by T cells of the helper phenotype imposed by the structural properties of the antigen that influence T cell receptor recognition, MHC restriction and B cell/T cell reciprocity. We wish to develop a biophysical approach to the problem using pigeon cytochrome as a model protein antigen, which we will apply to other protein antigens in later years of the tenure of this grant. There are three specific aims of this proposal. Firstly, we will extend our studies on the role of conformation in T helper cell recognition of processed protein antigens to molecular environments that are more analogous to the site of recognition in vivo. Thus, by comparing the efficacy of peptide analogues, in T cell proliferation assays and measuring their conformational properties in artificial membranes, we hope to reduce the biological phenomena of T cell determinant recognition of protein antigens to the molecular level. Secondly, we will determine the structural features of T cell recognition that are attributable to factors other than direct interaction with the T cell receptor using a biophysical approach to measuring Ia/peptide interactions, and the interaction of antigenic peptides with the antigen presenting cell membrane. In addition we will use protein engineering to manipulate the internal structure of the cytochrome c molecule in order to address the important question of how much antigen processing is necessary for recognition of this molecule. Finally, we will investigate the role of B cell/T cell collaboration in shaping the B cell and T cell repertoire. We will investigate the limits to the B cell response imposed by T helper cell specificity by using the technique of transfer fusion to manipulate the specificities of the T cell populations available to help transferred B cells. We will also study the ability of T cells to respond to antigen presented by B cells expressing receptors of varying specificity.